1. Field of the Invention
The present invention relates generally to a system for controlling and monitoring the temperature of heaters, and more particularly, to a heater control system for a plurality of individual pipe heaters positioned adjacent each other on a pipe with a like number of controller modules, which are configured for daisy chain connection together and for individual connection to individual mounting on and pipe heaters to provide individual electronic temperature and power control at each of the pipe heaters.
2. Description of the Related Art
The use of pipe heaters is widespread in semiconductor manufacturing, chemical, and pharmaceutical processing, plastics manufacturing, food processing, and other industries to heat piping systems to control various production and waste processes. Typically, the temperature of the piping system must be kept within a certain temperature range to keep gases or liquids flowing in the pipes at desired temperature levels as they are transported from one place to another. For example, in the semiconductor manufacturing industry, flexible insulated heaters, such as those disclosed in U.S. Pat. No. 5,714,738 to Hauschulz et al., are installed along the length of piping and piping components downstream from a reaction or deposition chamber to maintain transported effluent gases and vapors within specific temperature ranges that prevent the effluent gases and vapors from reacting, condensing, or depositing and building up solids on inside pipe walls, in valves, and in other pipe components before they can be trapped and removed in a cost effective manner.
In many industrial applications, the acceptable temperature range for the piping is tight or small, i.e., within a few degrees of a set point, and sometimes, the set point temperature is relatively high, e.g., above 180° C. Also, some pipes are fairly long and heat transfer rates may vary in different locations, so individual control of numerous individual pipe heaters positioned along the length of a pipe is needed to prevent development of local hot spots or cold spots. Therefore, there is a significant demand for an accurate and responsive heater control system that allows the user to obtain and maintain temperatures of piping components within user selectable ranges, including capability of controlling individual pipe heaters to deliver different heating power to various pipe locations as needed to maintain a desired temperature profile. Further, because consequences of individual heater failures can be quite expensive due to down time for the manufacturing process to disassemble and clean or replace clogged or damaged pipes, valves, and other components, heater control systems should be able to provide the user with operating information during use, such as whether the heater is “on” or “off” and whether the heater is within a specified temperature range. Pipe heaters often have to be installed on piping components that are small, such as 2-inch or smaller diameter piping, and in places where there is little or no clearance between piping components and adjacent structures. Therefore, users of the heaters often need heaters and associated control equipment that is not bulky or difficult to install, that is durable enough for industrial use, and that is easy to maintain and/or replace. Of course, the heaters and heater control systems must be configured to meet any and all safety standards (e.g., electrical and fire safety standards) that may apply to the particular industry.
One approach that is currently used to provide pipe heater control is to use an individual, self-contained, electro-mechanical temperature controller installed on each heater. With respect to pipeline heaters, these electro-mechanical temperature controllers are typically either bimetallic snap-action or creep-action thermostats, which are generally compact in size and relatively inexpensive. Unfortunately, such temperature controllers that utilize bimetallic or other snap-action or creep-action type thermostats generally have a single, fixed temperature set point and provide only limited temperature control.
In this regard, most snap-action electro-mechanical temperature controllers have a 15° C. or larger hysteresis or deadband around a set point temperature, which is unacceptable for applications that require tight pipe temperature control within only a few degrees of set point. Creep-action thermostats offer tighter initial temperature control, but they then become inaccurate as they drift over time. They also have short service lives due to high levels of electric arcing that occurs between their switch contacts. Also both of these types of electro-mechanical temperature controllers must be configured and installed such that there is intimate thermal contact with the active heater surface of the pipeline heater to function properly. Therefore, the general practice is to permanently embed the electro-mechanical temperature controller within the pipeline heater, and when the thermostat fails or needs servicing, the entire heater with controller must be replaced and typically scrapped. Another problem with most electro-mechanical temperature controllers is that they provide little or no operating information during use, and to find a non-functioning heater, operating or maintenance personnel have to touch each of the heaters with their hands to determine if it is warm and therefore, presumably operating. Additionally, the users of these heaters often are left without any accurate information on the actual operating temperature of the heater.
Another approach to heater control for pipes is the use of electronic temperature controllers that are positioned remote from the heaters and communicate via numerous individual data and power lines with extending from the remote controller to each heater. While such electronic temperature controllers, when combined with thermocouples, provide improved control of each heater and a tighter temperature range, they are relatively costly, and the large bundles of wires are cumbersome to install, especially in tight spaces. The high cost per controller and tangle of wires has led many users to bundle several heaters together in a zone or piping portion and to place the entire zone under the control of a single controller. While that solution decreases the complexity and tangle of wires, it also results in all the pipe heaters in a zone being set to a single temperature and, of course, the accuracy of control decreases with the overall size of the zone. For example, such a zone typically comprises one master and one or more slave heaters. The temperature sensor used by the single electronic controller is located near or connected to the master heater, and the temperature sensed at this single point in the piping system drives the heater control for all the heaters in the zone. However, for a particular required thermal loading, i.e., heating power needed, a temperature profile may be, and often is, different at each of the individual slave heater locations. Also, there is no way to ensure that individual slave heaters in a zone do not run arbitrarily hotter or colder than the master heater, which leads to decreased accuracy or tightness in controlling the temperature throughout the piping system or zone.
The use of a single controller to operate an entire zone may also create safety issues. For example, if the master heater fails cold or low, the controller typically operates or controls the other properly operating heaters in the zone to run hotter and overheat the rest of the piping system. In other words, if the slave heaters are not properly controlled within the zone, and thermal “run away” can result in blown fuses and/or fires, which cause safety hazards and significant down time within the manufacturing facility.
Additionally, the central controllers for systems in which a central controller is wired to control many individual heaters are relatively large, e.g., 48 mm by 96 mm by 100 mm and must be located remote from the heaters due to space and mounting constraints within the typical industrial setting. The size of each central controller becomes more of a problem in practice because a protective cage is often placed around the controller to protect sensitive electronic components from inadvertent damage from high temperature sources and physical contact. Further, installation and maintenance of the remotely-located central controllers for a large number of individual heaters are problematic because of the number of wires that must be run between the central controller and each heater. These wires generally include a power supply line for providing AC power to each heater from the controllers and a temperature sensor line to connect the controller to the thermocouple or other temperature sensing device. For safety and convenience, these wires are often strapped or bundled together, which makes it harder for maintenance personnel to work on a single heater, yet unbundling leaves an even more undesirable tangle of wires. Such “rat's nest” of wiring in the piping system makes maintenance, upgrading, and troubleshooting of these heater control systems time consuming and difficult for operating and maintenance personnel.
Consequently, there remains a need for an improved heater control system for providing enhanced control and monitoring of individual heaters in pipe heating systems, but without the concomitant wiring and controller complexity and physical size that is typically associated with such systems.